1. Field of the Invention
The present invention relates to a method and an apparatus for detecting patterns such as reticles and masks used for the fabrication of semiconductor devices having a high degree of integration such as LSI's. More specifically, the invention relates to a method and an apparatus for detecting abnormal patterns suited for discriminating and detecting tiny foreign matter (inclusive of pattern defects) adhered on the reticles and masks from the pattern edges.
2. Description of the Prior Art
The prior art will now be described with reference to the case of detecting tiny abnormal patterns on the reticles in the process for fabricating LSI's which is a representative technology to which the present invention is adapted.
In the exposure step used for the fabrication of LSI's, a chrome pattern on a thick plate called a reticle is printed by baking onto a semiconductor wafer. When foreign matter and defects exist on the reticle in this step, the pattern is not correctly printed onto the semiconductor wafer, and all of the LSI chips become defective. It is therefore essential to inspect for foreign matter and defects prior to the exposure from the standpoint of controlling the quality of reticles.
In recent years, smaller foreign matter is imposing another problem as the wiring patterns become more fine accompanying the increase in the integration degree of LSI's. In preparing the reticles, furthermore, the resist remains, chromium or chromium oxide for pattern formation remains after etching, and impurities dissolved in the reticle wash liquid coagulate at the time of wash and dry, imposing a problem of formation of foreign contaminant film which is on the increase.
A conventional apparatus for inspecting foreign matter and defects consists, as disclosed, for example, in Japanese Patent Laid-Open No. 65428 /1984, of means for illuminating a substrate by a laser beam from a tilted direction, a first lens which is provided above the substrate so that an illuminating point of the laser beam and a plane of focal point are nearly in agreement with each other, and which focuses the scattering light of a laser beam, a shutter which is provided on a Fourier transformed plane of the first lens and which shuts off the regularly scattered light from the substrate pattern, a second lens which subjects the scattered light from foreign matter obtained through the shutter to the inverse Fourier transformation, a slit which is provided at the imaging point of the second lens to shut off the scattered light from the areas other than the laser beam illuminating point on the substrate, and a light receiver which receives the scattering light from foreign matter that has passed through the slit.
According to the above apparatus, attention is given to the fact that a pattern is generally constituted in the same direction or by a combination of two to three directions in a field. The light diffracted by the pattern is removed by a space filter disposed on the Fourier transformed plane in order to emphasize and detect only the light that is reflected by foreign matter.
There has further been proposed a method for detecting defects by comparing the data of an inspected reticle detected by using an optical system with the data of a standard reticle detected using said system or with design data, as disclosed, for example, in Japanese Patent Laid-Open No. 139278/1983.
According to Japanese Patent Laid-Open No. 65428/1984 that pertains to the prior art as described above, the light reflected by foreign matter is split by the shutter from the light reflected by the pattern, the light reflected by foreign matter is detected by the slit, and the foreign matter is detected by the binary method contributing to simplifying the detecting mechanism. However, the foreign matter is detected by an indirect illumination, i.e., illumination by laser beam from an upper tilted direction, which is different from the traditional exposure system. Namely, only the light reflected by the chrome pattern of a particular angle is shut off, and foreign matter is not discriminated relying upon the whole chrome pattern.
When foreign matter is detected by the indirect means as described above, even foreign matter having no real problem (hereinafter referred to as lie detecting) is detected. In particular, when the foreign matter increases with the decrease in the size of the patterns, the foreign matter that has problems increases, too, though the problems may not yet be regarded as real problems. Therefore, the number of lie detections increases, and an increased amount of work is required for checking, analyzing and removing foreign matter, detrimental to operation efficiency.
Next, Japanese Patent Laid-Open No. 139728/1983 that also pertains to the aforementioned prior art has an optical system similar to the exposure system, making it possible to simply constitute the optical system compared with that of the aforementioned prior art. However, the image signal processing system for comparing the data is complex compared with that of the aforementioned prior art and requires an extended period of time for inspection.
Furthermore, a variety of devices have been developed for detecting foreign matter adhered onto the reticles and masks. According to the prior art disclosed in Japanese Patent Laid-Open No. 65428/1984, the reticle is directly illuminated by a polarized laser at a predetermined angle of incidence, and the foreign matter is discriminated by utilizing the fact that the foreign matter has a direction of polarization in the reflected light different from those of the reticle substrate and the pattern. According to the prior art disclosed in Japanese Patent Laid-Open No. 101390/1984, attention is given to the fact that the pattern edge on the reticle is generally in the same direction or consists of a combination of two to three directions in the field. Further, the diffracted light caused by the pattern edge is removed by a space filter disposed on the Fourier transformed plane, to emphasize and detect only the light that is reflected by the foreign matter.
According to the prior apparatus disclosed in Japanese Patent Laid-Open No. 186324/1984, attention is given to the fact that the scattered light caused by the pattern edge has directivity but the scattered light caused by foreign matter has no directivity, and the foreign matter is discriminated relying upon the logical product of light quantities received by the light-receiving elements installed at a plurality of places.
According to the prior apparatuses disclosed in Japanese Patent Laid-Open Nos. 154634/1985 and 154635/1985, the foreign matter is discriminated by arranging a plurality of detectors utilizing the phenomenon in that the diffracted light caused by the pattern edge is focused in a predetermined direction only whereas the scattered light caused by foreign matter is scattered in all directions.
In the manufacture of semiconductor devices such as LSI's, a pattern on a master plate called reticle is printed by baking onto a semiconductor wafer in the step of exposure. In this case, if foreign matter exists on the reticle, the pattern is not correctly printed and all of the chips become defective. It is therefore necessary to detect foreign matter prior to effecting the baking from the standpoint of controlling the quality of reticles.
In producing the reticles and masks, however, the residue of resist, the remnants of chromium or chromium oxide for pattern formation after the etching, and impurities that are dissolved in the reticle wash liquid and that coagulate at the time of washing and drying the reticle, adhere onto the reticle. However, such foreign matters were so tiny and formed a thin contaminant film that there seldom arouse any problem thus far. As the technology for highly densely integrating the LSI's advances and as the wiring patterns become more fine, however, the presence of foreign matter of the order of submicrons becomes a serious problem though that was not so far regarded as a problem.
According to the conventional apparatus disclosed in Japanese Patent Laid-Open No. 65428/1979, however, the light reflected by tiny foreign particles and thin contaminant films is so weak compared with the light reflected by the pattern edge that it is not possible to distinguish the pattern edge over the tiny foreign particles and thin contaminant films. The light reflected by the tiny foreign particles can be emphasized utilizing the conventional technology disclosed in Japanese Patent Laid-Open No. 101390/1984. However, limitation is imposed on the light reflected by the pattern edge that can be erased, and it is impossible to remove all of the light reflected by the pattern edge using the same space filter.
According to any other prior technology, it is not allowed to distinguish tiny foreign particles of smaller than 1 .mu.m and thin contaminant films over the pattern edges. This is because, the light is illuminated over a wide area according to the conventional art, and the sum of strength of the scattered light caused by the pattern edge so increases that the signals of scattered light caused by tiny foreign matter become undistinguishable. Even if the numerical aperture of the lens is increased in order to avoid such adverse effects, it is not allowed to distinguish the pattern edge over the foreign matter with the conventional apparatus. This is because, if the numerical aperture of the lens is great, the light is illuminated from many directions whereby the light scattered by the pattern edge loses directivity making it difficult to distinguish the light over the light scattered by foreign matter. That is, limitation is imposed on reducing the illuminated region based upon the method which illuminates one point.